x^2-4x+p^2-8p+4=0
x=-2代入
4+8+p^2-8p+4=0
p^2-8p+16=0
(p-4)^2=0
p=4代入原方程
x^2-4x+16-32+4=0
x^2-4x-12=0
(x+2)(x-6)=0
x1=-2,x2=6